(1) Field of the Invention
This invention relates to cooling assemblies and, in particular, to refrigerated detector units including a cryostat, for detecting an electromagnetic variable.
(2) Prior Art
Infrared detectors for use in such applications as missiles guidance are known in the prior art. For example, the Chaparral and Sidewinder missiles both use refrigerated detector units to sense infrared radiation used to guide the missile. Cooling of the infrared detector is necessary because the infrared detector functions only at about 77.degree. Kelvin.
In some cryostat heat transfer devices for cooling the infrared detector, a material such as air or nitrogen is supplied at a relatively high pressure, for example, at 3000 pounds per square inch. The coolant is supplied in a gaseous state to the cryostat wherein the adiabatic expansion of the coolant changes the coolant from a gaseous to a liquid state. The low temperature of the coolant lowers the temperature of the infrared detector. Heat is also absorbed as the coolant subsequently changes from a liquid back to a gaseous state thereby cooling the incoming gas.
The construction of prior art refrigerated detector unit cryostat assemblies has utilized a thermos type or dewar flask wherein a vacuum is created between an inner and an outer wall. Typically, the coolant is introduced into a generally cylindrical recess defined by the inner wall and the infrared device is mounted in the vacuum on the inner wall. Also, the inside of the inner wall and the outside of the outer wall are coated with a metallic, conductive material such as silver. This material is grounded and acts as a discharge path for any electrostatic charge on the coolant gas. Such electrostatic charge on the coolant gas is undesirable because it contributes electrostatic noise to the infrared detector and hinders detection of infrared radiation.
In addition to reducing electrostatic noise, a silver coating also shields the detector from various acoustical or microphonic noises such as caused by the impact of the coolant on the inner wall. In particular, the coolant is discharged into the recess from a coolant tube positioned along a radius of the generally cylindrical recess and is directed at the cylindrical wall of the recess. The coolant striking against the cylindrical wall causes the microphonic or acoustical noise which adversely affects the performance of the infrared detector by increasing the noise in the detected signal.
Further, a silver coating on the glass also reduces the noise of cryostat cycling. The cryostat cycling occurs when a temperature sensing device in the cryostat senses that a valve needs to be opened to admit coolant into the recess adjacent the infrared sensor to cool the sensor. The opening and the closing of the valve admitting the gas causes an undesirable cycling noise which is picked up by the infrared detector.
Although the silver coating on the inside of the inner wall and the outside of the outer wall performs a useful function, the inner coating is particularly disadvantageous because of the difficulty in applying a uniform silver coating, the costs of the process, and an undesirable large loss of product when fabricating a dewar flask assembly. Indeed, not only is the actual fabrication of the silver coated inner glass difficult, but the thorough testing of a completed dewar flask assembly, such as is required for use in a missile, sometimes produces thermal and/or mechanical stresses sufficient to cause breakage or degradation of the silver coated glass wall. The silver used has glass particles in its composition. When fired on, the silver actually bonds and becomes part of the glass wall. The thermal coefficients of expansion of the glass and silver are very close. It is believed that the strength of the glass lies at the surface and silver migration weakens this glass strength.
Because of the difficulty and cost of producing and testing such silver coated glass, it would be desirable to eliminate the need for silver coated glass. However, removal of the silver coating must not increase the low level of electrostatic noise, cycling noise and of acoustical or microphonic noise.